Variances in process, temperature, and/or voltage may affect operational characteristics of the circuits fabricated by the processing. Prior art methods and circuits often use negative and positive temperature effect current sources to bias a detection circuit to ameliorate variances caused by process used, temperature, and/or voltage variations. This, however, often focuses only on temperature compensation and/or on voltage tracking. Often a bipolar device is used to generate a positive temperature effect current source.
In the described examples, the positive supply voltage is designated as supply voltage VDD and the ground supply voltage, having a nominal value of 0 Volts is designated as supply voltage VSS.
FIG. 1 is a representation of a prior art DRAM memory cell. The memory cell includes a p-channel access transistor 101 and a p-channel storage transistor 102 that is configured as a storage capacitor. The gate of the access transistor 101 is connected to word line 103 and the drain of the access transistor 101 is connected to the bit line 105. The source of access transistor 101 is coupled to the source region of transistor 102. The gate of transistor 102 is connected to receive a negative boosted bias voltage VBB. The Bias voltage VBB is limited by the break down voltage of the gate oxide of capacitor 102 and the highest voltage stored on the electrode. In general, bias voltage VBB is set to a voltage that is greater V1 minus Vbd. In general V1 is equal to the positive supply voltage VDD. N-well is biased to a boosted positive voltage VPP that is greater than the VDD supply voltage by a voltage that is approximately equal to the absolute value of the threshold voltage of p-channel access transistor 101.
Certain voltage levels, e.g. VPP, need to be at desired levels in order for the circuit (FIG. 1, for example) to work correctly. VPP needs to be at a level that will turn off a transfer gate well, VBB needs to be deep enough to turn on transfer gates, and so on. However, at fast corner processing, for example, or at high temperature, threshold voltages may shrink, meaning a gate may be turned on with a shallower voltage VBB but may need a greater voltage VPP to turn off the gate. The inverse may be true at slow corner processing. Therefore, VPP often needs to track VDD variations, e.g. maintain a constant voltage differential.
Table 1 is an example of desired VPP and VBB levels for variation in operations, temperature vs. baseline conditions which reflect the events above.
TABLE 1ConditionsDesired VPPDesired VBBBaseline @ 25° C., VDDVPPVBBProcessSlow cornerDecrease VPPIncrease VBBFast cornerIncrease VPPDecrease VBBTemperature@ 125° C.Increase VPPDecrease VBB@−40° C.Decrease VPPIncrease VBBSupply Voltage+20% VDD+20% VDDVBB−20% VDD−20% VDDVBB
A problem with the prior art is that the methods and circuits used to compensate for process-voltage-temperature (PVT) variations cannot reflect real world demands placed in word line voltage in the existence of process variations. For example, the prior art methods and circuits cannot work for a low voltage process because of the numerous components, e.g. two MOS transistors, a resistor, and a bipolar transistor connected in series between VDD and VSS voltage sources for positive temperature effect current source.
Further, often the prior art maintains voltages at set levels, e.g. VPP=VDD+|VTP| and VBB=VSS−|VTP|. If the threshold voltage VTP becomes small, such as from environmental changes, off-currents may increase due to a smaller turn off voltage operated in a leaky condition. FIGS. 2 through 4 are representative of prior art solutions.
FIG. 2 is a prior art VPP detection circuit which uses INTE and IPTE current sources. FIGS. 3a and 3b are prior art current sources associated with prior art solutions, such as shown in FIGS. 2 and 4. FIG. 3a shows a prior art circuit for INTE and FIG. 3b shows a prior art circuit for IPTE where INTE=|VTP|/R is negative and IPTE=kT/q×Ln(m) is positive. FIG. 4 is a prior art VBB detection circuit. These prior art solutions use negative and positive temperature effect currents sources to bias the detection circuit and only focus on temperature and voltage tracking and thus cannot reflect the real demand on word line voltage from process variations.
Therefore in order to obviate the deficiencies in the prior art, it is an object of the present disclosure to provide a circuit that provides desirables word line voltage levels VPP and VBB over P.V.T variations.
These objects and other advantages of the disclosed subject matter will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal or the claims, the appended drawings, and the following detailed description of the preferred embodiments.